PROJECT ABSTRACT To date, 33 states have implemented prescription duration limits to combat the national opioid epidemic. The intention of these policies is to minimize patient exposure to opioids, thereby reducing dependence rates, overdoses, and costly health care utilization. However, limiting prescription supply may have unintended adverse consequences, including increased health care use or suicide due to uncontrolled pain, and use of illicit non-prescription substances (e.g., heroin, illegally manufactured fentanyl) as a substitute for legally-prescribed opioids. Despite these competing potential impacts, no study has assessed the impact of a prescription duration limit policy. The proposed study will be the first to measure the impact of these policies on prescription patterns (Aim 1), health care use (Aim 2), and health outcomes (Aim 3). Crucially, we will assess both intended and unintended ? yet potentially dire ? impacts of these policies. We will address this important knowledge gap by using an innovative ?dose response? event-study methodology. In particular, we will compare outcomes in areas (counties) more ?exposed? to the policies to those in less exposed areas. We define relative exposure as the rate of opioid prescriptions that exceeded the policies? limit on days? supply, prior to policy implementation. This approach has an important advantage over other methods: we are able to distinguish between metrics that will be disproportionately targeted by duration limits (which is based specifically on days? supply) versus other opioid-specific metrics (e.g., number of prescriptions), which are likely to be influenced by other opioid reduction policies. This allows us to isolate the impact of duration limits alone, unlike other methodological approaches, which may be biased due to the many other opioid-related policies that have been implemented. Data for the study come from West Virginia (WV) and Kentucky (KY) Medicaid claims. Because WV and KY have different duration limits, we will be able to directly compare the impact of stricter versus more lenient policies. Results from this study will provide urgently-needed evidence that will inform public health policy and clinical practice across the US. This study is firmly aligned with NIDA?s research emphasis on provider and health systems-based approaches to drug abuse prevention.